Ink-jet printing systems commonly utilize either direct printing or offset printing architecture. In a typical direct printing system, ink is ejected from jets in the print head directly onto the final receiving medium. In an offset printing system, the print head jets the ink onto an intermediate transfer surface, such as a liquid layer on a drum. The final receiving medium is then brought into contact with the intermediate transfer surface and the ink image is transferred and fused or fixed to the medium.
In some direct and offset printing systems, the print head moves relative to the final receiving medium or the intermediate transfer surface in two dimensions as the print head jets are fired. Typically, the print head is translated along an X-axis while the final receiving medium/intermediate transfer surface is moved along a Y-axis. In this manner, the print head "scans" over the print medium and forms a dot-matrix image by selectively depositing ink drops at specific locations on the medium.
With reference now to the image deposition process in an offset printing architecture, the print head moves in an X-axis direction that is parallel to the intermediate transfer surface as the drum supporting the surface is rotated. Typically, the print head includes multiple jets configured in a linear array to print a set of scan lines on the intermediate transfer surface with each drum rotation. Precise placement of the scan lines is necessary to meet image resolution requirements and to avoid producing undesired printing artifacts, such as banding and streaking. Accordingly, the X-axis (head translation) and Y-axis (drum rotation) motions must be carefully coordinated with the firing of the jets to insure proper scan line placement.
As the size of the desired image increases, the X-axis movement/head translation and/or Y-axis motion requirements become greater. One technique for printing larger-format images is disclosed in co-pending application Ser. No. 08/509,844 for INTERLEAVED INTERLACED IMAGING, assigned to the assignee of the present application. This application discloses a method for interleaving or stitching together multiple image portions to form a larger composite image. Each of the image portions is deposited with a separate X-axis translation of the print head. After the deposition of each image portion, the print head is moved without firing the jets to the start position for the next image portion. Adjacent image portions overlap and are interleaved at a seam to form the composite image.
In this image deposition method, the relative position of each image portion must be carefully controlled to avoid visible artifacts at the seam joining adjacent image portions. With specific regard to the X-axis movement of the print head, it is necessary to precisely deposit each image portion such that adjacent image portions are aligned to properly interleave at the seam. Furthermore, the X-axis movement must be capable of repeatably producing composite images having different sizes and positions on the print medium without creating a visible artifact at the seam between adjacent image portions. Thus, an accurate X-axis positioning mechanism and corresponding positioning method are required.
Prior art ink jet printers have utilized various mechanisms to impart X-axis movement to a print head. An exemplary patent directed to an X-axis positioning mechanism is U.S. Pat. No. 5,488,396 for PRINTER PRINT HEAD POSITIONING APPARATUS AND METHOD (the '396 patent), assigned to the assignee of the present application. This patent discloses a motion mechanism comprising a stepper motor that is coupled by a metal band to a lever arm. Rotation of the lever arm imparts lateral X-axis motion to a positioning shaft that is attached to the print head. This mechanism translates each step of the stepper motor into one pixel of lateral X-axis movement of the print head. The amount of X-axis translation per step of the stepper motor is adjustable by an eccentrically mounted ball that is positionable on the lever arm.
While the positioning mechanism of the '396 patent provides highly accurate and repeatable positioning of a print head, it is nevertheless subject to minor displacement errors arising from such factors as imbalances in stepper motor phase and thermal expansion of various components under changing operating temperatures. Additionally, variations in horizontal jet spacing on the print head can create uncertainty as to the actual X-axis position of a jet, and thus uncertainty in the placement of certain scan lines. Furthermore, when the above described method for printing an interleaved composite image is used, these types of displacement errors are magnified at the seam joining the two image portions. Even very slight deviations in scan line placement on the order of 0.0003 inches (0.0076 mm), normally imperceptible within a fully interlaced image, generate a visible artifact due to misalignment at the seam.
Accordingly, the present invention is directed to a print head positioning method that substantially eliminates displacement errors arising from mechanical variations in a print head positioning mechanism.